1. Field of the Invention
The invention which relates to the continuous burial of a pipe in a trench, more particularly in an undersea bed formed of materials having low cohesion, relates to a method and device for temporarily supporting the walls during the trench digging operation.
2. Description of the Prior Art
For protecting pipes, and particularly oil pipe lines, from the harmful effects of undersea currents or swell, as well as for preventing sea anchors or fishing nets from fouling these pipes, efforts have been made for numerous years to bury these pipes in the undersea beds. This has been achieved by disposing these pipes in trenches previously dug in these undersea beds.
This has also been achieved by digging a trench under a pipe previously disposed on the sea bed and which then rests, under the effect of its own weight, by flexion on the bottom of the trench at a certain distance behind the digging devices which are moved along the pipe thereunder.
But although the use and implementation of different devices allow such trenches to be formed and although a pipe can be buried without too much difficulty when the underwater bed has sufficient cohesion for ensuring the stability of the vertical or substantially vertical walls of the trench, the same cannot be said when the undersea bed is sandy, muddy or formed of other powdery materials or materials without sufficient cohesion, for the side walls collapse and the trench fills in, at least partially, before the pipe is suitably positioned on the original bed thereof.
In this latter case, to prevent immediate or substantially immediate filling in of the trench, using different devices a depression or valley of substantial width is formed in the undersea bed, which on the one hand means moving very voluminous amounts of materials and, on the other hand, does not obtain the desired protection for the pipe, except by filling in the depression laterally using the removed materials, which substantially increases the cost of the operation.
One known solution consists then in reinforcing the walls of the trench by means of coffering or formwork which must then be removed and shifted after lowering the pipe, which is a complicated operation.
Another technique consists in fluidifying materials by injecting water under the pipe, which then descends under its own weight.
This delicte technique can however only be used for clean sands.
A new technique used more recently consists in substituting a compressible body of the envelope kind made from a flexible and deformable material for the materials removed from the trench, progressively as this latter advances, so that this body, by its presence or by the pressure which it exerts on the side walls of the trench prevents these latter from collapsing. The reduction of volume of this body is then controlled so as to allow the pipe to be lowered and positioned in the trench and so as to cause collapsing of the side walls and possibly the filling thereof.
According to this method, if required, as the trench advances means are displaced for temporarily holding its side walls in position, preventing collapse thereof before the compressible body has been positioned in the trench.
The flexible and deformable material envelope is brought progressively and continuously into the trench, and is then filled with fluid to a pressure higher than by hydrostatic pressure; by the pressure which it then exerts on the side walls of the trench it prevents the temporary collapsing thereof. The pipe to be buried rests on this envelope usually provided with holes. The progressive reduction of the volume during leaking through the holes of the fluid which it contains cause the pipe to be lowered to the bottom of the trench, thus allowing the side walls thereof to collapse and the pipe to be covered.
This method involves consequently the continuous provision of a flexible and deformable envelope which remains buried under the covering materials and under the pipe after laying thereof and which is therefore lost. Apart from the cost of such an envelope which is therefore to be provided over a length equivalent to that of the pipe, considerable means must also be provided, embarked on a follower ship, for feeding this amount of previously stored envelope, as the digging means advance, and for injecting the pressurized fluid therein when it is resting at the bottom of the trench.
In addition, so that such burial in accordance with this technique takes place under acceptable working conditions, a certain number of precautions must obviously be taken. It is in fact necessary, during the advance of digging of the trench followed by positioning and filling of the flexible envelope, for the pipe to be lowered correctly and progressively with such advance at a certain distance behind the digging zone. In fact, if such lowering were too rapid, there could be an excessive curvature of the pipe under the effect of gravity with a consequent risk of breakage. On the other hand, in some other cases, since the weight of the pipe itself filled with water is insufficient this pipe will descend much too slowly into the trench, the volume of fluid then having to fill the flexible envelope over a greater length; the result is on overconsumption of water through the leak orifices, and great difficulty in regulating the pressure throughout the whole envelope. Since the fluid pressure in said envelope cannot then be correctly regulated, the profile of the pipe can no longer be adjusted and even in some cases the maintenance of a sufficient pressure for preventing collapse of the trench is no longer possible. This problem of regulating the fluid supply pressure of the envelope and the leak rate therefrom through appropriate openings is difficult to overcome, because the continuous flexible envelope has an infinite volume.
The invention brings an efficient solution to these problems for it provides, no longer an open envelope of infinite volume having leak openings, but a closed envelope, of finite volume, whose very design means that it ensures temporary maintenance of the walls of the trench, facilitates lowering into this latter of the pipe to be buried while moving without rubbing against the bottom of the trench, without for all that modifying the mass of the initial fluid contained in the envelope.